15 Working with Pre-Built Libraries

15.1 Introduction

Some library vendors do not release source code for their software but do release pre-built binaries. When shared libraries are built, they should be versioned (see this article for some background), but sometimes this is not done.

To summarize, a versioned library must meet two conditions:

  1. The filename must have the version appended, for example: libfoo.so.1.2.3.

  2. The library must have the ELF tag SONAME set to the major version of the library, for example: libfoo.so.1. You can check this by running readelf -d filename | grep SONAME.

This section shows how to deal with both versioned and unversioned pre-built libraries.

15.2 Versioned Libraries

In this example we work with pre-built libraries for the FT4222H USB I/O chip. Libraries are built for several target architecture variants and packaged in an archive as follows:

├── build-arm-hisiv300
│   └── libft4222.so.
├── build-arm-v5-sf
│   └── libft4222.so.
├── build-arm-v6-hf
│   └── libft4222.so.
├── build-arm-v7-hf
│   └── libft4222.so.
├── build-arm-v8
│   └── libft4222.so.
├── build-i386
│   └── libft4222.so.
├── build-i486
│   └── libft4222.so.
├── build-mips-eglibc-hf
│   └── libft4222.so.
├── build-pentium
│   └── libft4222.so.
├── build-x86_64
│   └── libft4222.so.
├── examples
│   ├── get-version.c
│   ├── i2cm.c
│   ├── spim.c
│   └── spis.c
├── ftd2xx.h
├── install4222.sh
├── libft4222.h
├── ReadMe.txt
└── WinTypes.h

To write a recipe to use such a library in your system:

  • The vendor will probably have a proprietary licence, so set LICENSE_FLAGS in your recipe.

  • The vendor provides a tarball containing libraries so set SRC_URI appropriately.

  • Set COMPATIBLE_HOST so that the recipe cannot be used with an unsupported architecture. In the following example, we only support the 32 and 64 bit variants of the x86 architecture.

  • As the vendor provides versioned libraries, we can use oe_soinstall from utils to install the shared library and create symbolic links. If the vendor does not do this, we need to follow the non-versioned library guidelines in the next section.

  • As the vendor likely used LDFLAGS different from those in your Yocto Project build, disable the corresponding checks by adding ldflags to INSANE_SKIP.

  • The vendor will typically ship release builds without debugging symbols. Avoid errors by preventing the packaging task from stripping out the symbols and adding them to a separate debug package. This is done by setting the INHIBIT_ flags shown below.

The complete recipe would look like this:

SUMMARY = "FTDI FT4222H Library"
SECTION = "libs"

COMPATIBLE_HOST = "(i.86|x86_64).*-linux"

# Sources available in a .tgz file in .zip archive
# at https://ftdichip.com/wp-content/uploads/2021/01/libft4222-linux-
# Found on https://ftdichip.com/software-examples/ft4222h-software-examples/
# Since dealing with this particular type of archive is out of topic here,
# we use a local link.
SRC_URI = "file://libft4222-linux-${PV}.tgz"

S = "${WORKDIR}"

ARCH_DIR:x86-64 = "build-x86_64"
ARCH_DIR:i586 = "build-i386"
ARCH_DIR:i686 = "build-i386"

INSANE_SKIP:${PN} = "ldflags"

do_install () {
        install -m 0755 -d ${D}${libdir}
        oe_soinstall ${S}/${ARCH_DIR}/libft4222.so.${PV} ${D}${libdir}
        install -d ${D}${includedir}
        install -m 0755 ${S}/*.h ${D}${includedir}

If the precompiled binaries are not statically linked and have dependencies on other libraries, then by adding those libraries to DEPENDS, the linking can be examined and the appropriate RDEPENDS automatically added.

15.3 Non-Versioned Libraries

15.3.1 Some Background

Libraries in Linux systems are generally versioned so that it is possible to have multiple versions of the same library installed, which eases upgrades and support for older software. For example, suppose that in a versioned library, an actual library is called libfoo.so.1.2, a symbolic link named libfoo.so.1 points to libfoo.so.1.2, and a symbolic link named libfoo.so points to libfoo.so.1.2. Given these conditions, when you link a binary against a library, you typically provide the unversioned file name (i.e. -lfoo to the linker). However, the linker follows the symbolic link and actually links against the versioned filename. The unversioned symbolic link is only used at development time. Consequently, the library is packaged along with the headers in the development package ${PN}-dev along with the actual library and versioned symbolic links in ${PN}. Because versioned libraries are far more common than unversioned libraries, the default packaging rules assume versioned libraries.

15.3.2 Yocto Library Packaging Overview

It follows that packaging an unversioned library requires a bit of work in the recipe. By default, libfoo.so gets packaged into ${PN}-dev, which triggers a QA warning that a non-symlink library is in a -dev package, and binaries in the same recipe link to the library in ${PN}-dev, which triggers more QA warnings. To solve this problem, you need to package the unversioned library into ${PN} where it belongs. The following are the abridged default FILES variables in bitbake.conf:

SOLIBS = ".so.*"
FILES:${PN} = "... ${libdir}/lib*${SOLIBS} ..."
FILES_SOLIBSDEV ?= "... ${libdir}/lib*${SOLIBSDEV} ..."
FILES:${PN}-dev = "... ${FILES_SOLIBSDEV} ..."

SOLIBS defines a pattern that matches real shared object libraries. SOLIBSDEV matches the development form (unversioned symlink). These two variables are then used in FILES:${PN} and FILES:${PN}-dev, which puts the real libraries into ${PN} and the unversioned symbolic link into ${PN}-dev. To package unversioned libraries, you need to modify the variables in the recipe as follows:

SOLIBS = ".so"

The modifications cause the .so file to be the real library and unset FILES_SOLIBSDEV so that no libraries get packaged into ${PN}-dev. The changes are required because unless PACKAGES is changed, ${PN}-dev collects files before ${PN}. ${PN}-dev must not collect any of the files you want in ${PN}.

Finally, loadable modules, essentially unversioned libraries that are linked at runtime using dlopen() instead of at build time, should generally be installed in a private directory. However, if they are installed in ${libdir}, then the modules can be treated as unversioned libraries.

15.3.3 Example

The example below installs an unversioned x86-64 pre-built library named libfoo.so. The COMPATIBLE_HOST variable limits recipes to the x86-64 architecture while the INSANE_SKIP, INHIBIT_PACKAGE_STRIP and INHIBIT_SYSROOT_STRIP variables are all set as in the above versioned library example. The “magic” is setting the SOLIBS and FILES_SOLIBSDEV variables as explained above:

SUMMARY = "libfoo sample recipe"
SECTION = "libs"

SRC_URI = "file://libfoo.so"

COMPATIBLE_HOST = "x86_64.*-linux"

INSANE_SKIP:${PN} = "ldflags"
SOLIBS = ".so"

do_install () {
        install -d ${D}${libdir}
        install -m 0755 ${WORKDIR}/libfoo.so ${D}${libdir}